Many of the injection molding apparatus known today include one mold clamping mechanism and one injection molding machine, and they are constructed in such a manner that resin material is injected into an injection mold, clamped by the mold clamping mechanism, to obtain a molded part or workpiece. However, the known injection molding apparatus have limitations in terms of productivity. Thus, there have been proposed injection molding apparatus that include a plurality of mold clamping mechanisms and a plurality of injecting machines, one example of which is disclosed in Japanese Patent Application Laid-open Publication No. 2003-320545 that will hereinafter be referred to as “the relevant patent literature” (see particularly FIGS. 1 and 2 of the relevant patent literature).
The injection molding apparatus disclosed in the relevant patent literature includes, as shown in FIGS. 1 and 2 of the relevant patent literature, two injecting machines, two mold clamping mechanisms, one oil tank and one hydraulic pump. One of the injecting machines is driven by a hydraulic cylinder provided as an injecting cylinder, one of the mold clamping mechanisms is driven by a hydraulic cylinder provided as a mold clamping cylinder, the other of the injecting machines is driven by a hydraulic cylinder provided as another injecting cylinder, and the other of the mold clamping mechanisms is driven by a hydraulic cylinder provided as another mold clamping cylinder. Hydraulic pressure is supplied from the hydraulic pump to all of the above-mentioned four hydraulic cylinders.
Because the four hydraulic cylinders are sometimes driven simultaneously, the hydraulic pump has a capacity capable of simultaneously driving the four hydraulic cylinders. Therefore, the hydraulic pump would unavoidably become large in size and costly, but also an amount of electric power to be supplied to an electric motor for driving the hydraulic pump would increase and thus electricity cost would increase.
In actual operation of such an injection molding apparatus, all of the hydraulic cylinders are sometimes driven simultaneously, and only one or some of the hydraulic cylinders are sometimes driven. However, even in the case where only one or some of the hydraulic cylinders are driven, considerable electricity cost would be required for driving the large hydraulic pump. Therefore, there has been a demand for an improved technique capable of downsizing the hydraulic pump in order to achieve energy saving or conservation and cost reduction of the injection molding apparatus.